A wheel loader is run by drive wheels being driven from an engine as a driving source through a torque converter. In other words, an engine output is used for a traveling load. In addition, the engine also provides a driving source for a steering mechanism, and a work machine such as a loader or the like. In other words, the engine drives a steering hydraulic pump; a pressurized oil delivered from the steering hydraulic pump is supplied to a steering hydraulic cylinder; and according to this, the steering mechanism is operated. In addition, the engine also drives a loader hydraulic pump; pressurized oil delivered from the loader hydraulic pump is supplied to a loader hydraulic cylinder; and according to this, a loader is operated. For the steering hydraulic pump and the loader hydraulic pump, a fixed displacement hydraulic pump, which is fixed in displacement, is used. Thus, the engine output is used not only for the traveling load, but also for the working hydraulic pressure load.
Running speed of the wheel loader varies depending upon the amount of pedal depression for an accelerator pedal. In other words, according to the amount of pedal depression for the accelerator pedal, revolutions for the engine is changed, and according to that, the vehicle speed is changed.
And, the wheel loader encounters many opportunities to carry out working in the situation in which both the traveling load and the working hydraulic pressure load are imposed, for example, in the situation in which the loader is vertically moved while the wheel loader is being run, as compared to the other types of work vehicles, such as the hydraulic excavator, and the like.
Herein, for the hydraulic excavator, the invention with which the maximum absorption torque or the displacement for the variable displacement hydraulic pump is changed according to the various working modes has already become well-known on the various patent literature as exemplified below.
Patent literature 1: Japanese Patent Application Laid-Open No. 62-58033
Patent literature 2: Japanese Patent Publication No. 2711833